284 Mr. J. P. Harrison on Radiation and Vapour. 



There can be no doubt that the apparent variations in solar 

 radiation at the same altitudes of the sun is due to heat re- 

 ceived by our instruments from the counterradiation of cloud 

 and other matter. Precisely the same laws are in operation as 

 regards this secondary radiation by day, as Dr. Wells proved so 

 conclusively are at work by night, though the action in the case 

 of the solar-radiation thermometer is masked by a continuous 

 supply of heat from the direct rays of the sun. 



On the occasion above alluded to, it was assumed as an axiom 

 that transparent vapour absorbs less heat than cloud, or even 

 fog, and consequently exercises less influence over temperature. 

 Now Dr. Wells, whilst perfectly aware of the fact that radiation 

 from the air itself was very slight, had found that it nevertheless 

 occurred, though he attributed it, in the absence of experiment, 

 to the presence of particles of solid matter suspended in the at- 

 mosphere. Professor TyndalPs discovery of the absorptive pro- 

 perties of aqueous vapour shows the sagacity and accuracy of 

 Wells's conclusions, but in nowise alters the results of his inves- 

 tigation. It was found by both Prevost and Wells that on clear 

 nights the slightest cloud passing across the zenith at once 

 caused a considerable rise in the mercury of an exposed ther- 

 mometer, and stopped the deposit of dew. 



The question has recently been submitted by Colonel Strachey 

 to the test of numbers in the pages of this Magazine*; but his 

 Tables afford materials for showing that the fall in temperature 

 by night at Madras, which was considered to be owing to the 

 decrease in vapour-tension, is to be ascribed, according to Dr. 

 Wells's law, to the absence of cloud. 



As regards the effects of vapour or cloud in the higher regions 

 of the air, it may be gathered from somewhat conflicting evidence 

 that bodies of vapour, of more or less tenuity and extent, float 

 about in the drier atmosphere, and affect the temperature of the 

 air in much the same way relatively as in the case of clouds 

 nearer the earth. 



If this be so, however, it may be asked, how is it that so little 

 difference is found four or five miles high in the readings of the 

 exposed and shaded thermometers ? It appears to follow as a 

 matter of necessity that less difference would be found, provided 

 no floating cloud (however light) overshadowed the instruments. 

 But here we are met at once with the fact that a sufficient shelter 

 is supplied by the balloon itself. 



May it not be that in the recent balloon ascents there were 

 special circumstances which rendered it difficult to obtain high 

 readings of insolation ? 



* Phil. Mag. Julv 1866. 



